Fix flaky test `ObsoleteFiles`
Fix flaky test `ObsoleteFiles`

Summary: The test `ObsoleteFiles` failed occasionally on slow device. This problem appears on Travis CI several times. The reason is that we did not wait until compaction jobs are finished in the test, while in slower device the background jobs take longer time to finish.

Test Plan: Pass existing tests.

Reviewers: yiwu, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D61479

OverlordQ/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,facebook/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,wenduo/rocksdb,norton/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,tsheasha/rocksdb,wenduo/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,tsheasha/rocksdb,Andymic/rocksdb,facebook/rocksdb,wenduo/rocksdb,vmx/rocksdb,facebook/rocksdb,vmx/rocksdb,facebook/rocksdb,tsheasha/rocksdb,norton/rocksdb,SunguckLee/RocksDB,OverlordQ/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,norton/rocksdb,norton/rocksdb,Andymic/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,norton/rocksdb,facebook/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,Andymic/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,facebook/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,norton/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,norton/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,bbiao/rocksdb,vmx/rocksdb,norton/rocksdb